1. Field of the Invention
The present invention relates generally to dispersion compensating waveguide fibers, and particularly to such fibers that are cabled or otherwise buffered and incorporated into telecommunications links designed to operate over a range of wavelengths.
2. Technical Background
Telecommunication systems using high powered lasers, high data rate transmitters and receivers, and wavelength division multiplexing (WDM) technology require optical waveguide fiber having exceptionally low total dispersion and polarization mode dispersion (PMD). The term total dispersion means all linear dispersion, including material dispersion and waveguide dispersion. In the art, total dispersion is sometimes called chromatic dispersion. In addition, the waveguide fiber must have characteristics which essentially eliminate non-linear phenomena such as self phase modulation (SPM) and four wave mixing (FWM). SPM can be limited by lowering power density, for example by increasing the mode field diameter or the effective area of the waveguide fiber. FWM is controlled by operating in a wavelength range over which dispersion of the fiber is non-zero.
Due to this requirement of non-zero dispersion, communication systems become dispersion limited at the very high bit rates. Consequently, dispersion compensating modules are typically employed to compensate for dispersion accumulated over a transmission link operating in, for example, the 1550 nm wavelength window. In order for the compensating fiber employed in the module not to become a limiting factor in the link, it should have functional parameters at least similar to the waveguide fibers in the link being compensated. For example, it is desirable for the compensating fiber to have similar effective area, cut off wavelength, and attenuation in comparison to the waveguide fiber in the link being compensated.
This desired performance is difficult to achieve, however. Usually some compromises in the form of design tradeoffs must be made. In one compensating scheme, a fiber having a very large dispersion, of sign opposite that of the dispersion of the cabled fiber forming the link, is contained within a dispersion compensating module and inserted into the link. For example, a telecommunications link comprising positive dispersion cabled fiber can be deployed in a link having regenerator spacing of about 100 km. A dispersion compensating module comprising fiber having a total dispersion at 1550 nm of about xe2x88x9270 ps/nm-km to xe2x88x92100 ps/nm-km typically would serve to compensate for the positive dispersion accumulated in the link. The length of fiber in the dispersion compensating module should be short in order to keep the module size relatively small and to minimize the attenuation added to the link by the dispersion compensating module. The attenuation of the fiber in the dispersion compensating module is typically much higher than that of the cabled or otherwise buffered fiber forming the link. The dispersion compensating module is essentially another link component that does not serve as a part of the link length. That is, the module does not serve as a part of the length that bridges the distance between transmitter and receiver.
Many of the high performance transmission links make use of wavelength division multiplexing (WDM) to maximize the information capacity of the links. This places an added requirement on the fiber of the dispersion compensating module in that compensation is desired over an extended wavelength range. Dispersion compensation over a wavelength range can be accomplished by control of the slope of the compensating fiber. A desirable target range of wavelengths for compensation corresponds to the wavelength range over which the gain versus wavelength curve of optical amplifiers, present in the transmission link, is relatively constant.
A first aspect of the present invention is a dispersion compensating optical waveguide fiber having a core region surrounded by a clad layer. In systems that are bandwidth limited, the waveguide fiber is preferably operated in a wavelength range over which only a single mode is propagated. Because the fiber guides light through its length, at least a portion of the clad layer must have a refractive index lower than that of at least a portion of the core region. The core region of the compensating single mode fiber preferably includes at least three segments. Substantially centered about the symmetry line of the fiber is a central segment having a relative refractive index xcex940% in the range of about 0.6% to 1.1% . The central segment is surrounded by a first annular region of negative relative refractive index, which in turn is surrounded by a second annulus of positive relative index. The relative refractive index of the first annular segment xcex941% is more negative than about xe2x88x920.4%. The relative index of the second annular segment xcex942% is greater than 0. The waveguide fiber so configured provides a negative total dispersion and a total dispersion slope more negative than about xe2x88x920.2 ps/nm2-km over a pre-selected wavelength range that includes the wavelength 1550 nm.
In an embodiment of this aspect, the pre-selected wavelength range is about 1500 nm to 1700 nm and preferably 1520 nm to 1650 nm.
In a further embodiment of this aspect of the invention, the core region includes a third annular segment surrounding the second annular segment and having a relative index xcex943%xe2x89xa60.2%
In yet a further embodiment of this aspect, the core region includes a fourth annular segment having relative index xcex944% xe2x89xa70. More specifically, the relative index range of xcex942% is about xe2x88x920.4% to 0.5%, of xcex943% is about xe2x88x920.2% to 0.2%, and of xcex944% is about 0 to 0.6%.
In another embodiment of this first aspect of the invention, xcex940% is in the range of about 0.8% to 1.1%, xcex941% is more negative than xe2x88x920.5%, xcex942% is in the range of about xe2x88x920.1% to 0.1%, xcex943% is in the range of about xe2x88x920.1% to 0.1%, and xcex944% is in the range of about 0.3% to 0.5%.
For each of these embodiments, attenuation values at 1550 nm are no greater than about 0.34 dB/km.
A second aspect of the invention is an optical waveguide fiber link, preferably using wavelength division multiplexing, that incorporates cabled waveguide fiber optically coupled to a cabled total dispersion and total dispersion slope compensating waveguide fiber (cabled compensating fiber). The definition of cabled compensating fiber is used consistently herein. Compensating over a range of wavelengths is equivalent to compensating total dispersion slope. High performance, fully or partially compensated links typically employ single mode waveguide fiber. However, the invention is applicable to two or more modes propagating in a waveguide fiber links as well.
Referring to FIG. 3, the link includes transmitter 58, cabled waveguide fiber 60, cabled compensating fiber 62 and receiver 64. Cabled waveguide fiber 60, preferably a single mode transmission fiber, and cabled compensating fiber 62 are optically coupled end to end in series to form the link length. The transmitter and receiver are designed for use in wavelength division multiplexed links. In one embodiment of the invention, the compensating fiber is made in accord with the first aspect of the invention and is cabled or otherwise buffered to form a portion of the link. The term cabled waveguide fiber as used throughout this application refers to the plurality of structures known in the art that may include buffered, stranded, or jacketed optical waveguide fiber. These structures may be deployed or installed essentially in the same way as any other communications cable. The link is designed so that the cabled transmission fiber is interposed between the cabled compensating fiber and a transmitter optically coupled to the cabled fiber. The transmitter is capable of launching into the cabled fiber a plurality of wavelengths selected from the particular wavelength range of the multiplexed signals. A receiver capable of receiving the plurality of launched wavelengths is optically coupled to the remaining end of the cabled compensating waveguide fiber. The compensating cable, preferably made in accord with the first aspect of the invention, can be designed to provide an end to end total dispersion (the sum of the total dispersion of the cabled transmission waveguide and the cabled compensating waveguide) of less than about +/xe2x88x9210 ps/nm at each of the launched wavelengths. The cabled compensating fiber has a total dispersion of opposite sign relative to the cabled fiber. The dispersion sum is the algebraic sum of the cabled waveguide dispersion and the cabled compensating waveguide dispersion.
In an embodiment of this aspect, the cabled dispersion compensating waveguide fiber is designed to completely compensate the dispersion of the cabled waveguide fiber so that the link total dispersion is zero at each of the wavelengths propagated in the link.
In another embodiment, the cabled compensating fiber is designed to provide and end to end total link dispersion that is positive. A preferred embodiment is one in which the end to end link dispersion is no greater than about 4 ps/nm.
An alternative embodiment incorporates a cabled dispersion compensating waveguide that provides a negative end to end link total dispersion no more negative than about xe2x88x924ps/nm.
When the slope compensation is uniform across the pre-selected wavelength range, the individual dispersion sums are equal. Such a configuration allows for uniform performance among the link channels. Each channel can thus be used at substantially equivalent bit rates over equal lengths. In addition, the configuration is compatible with soliton signal multiplexing.
In each of the aspects and embodiments disclosed and described above, the pre-selected wavelength range may be from about 1500 nm to 1700 nm. Preferably, the pre-selected wavelength range is 1520 nm to 1650 nm.
This aspect of the invention results in a number of advantages over prior art dispersion compensating modules. By incorporating a cabled or otherwise buffered fiber as a part of the link length to compensate (cabled compensating fiber) at least partially for the link dispersion over a range of wavelengths, the requirements placed on the dispersion compensating module could be relaxed. The compensating module would be lower in cost, easier to manufacture, and shorter in length.
Further, some systems are designed to have incomplete dispersion and dispersion slope compensation, in which case the presence of the cabled compensating fiber in the link would obviate the need for the compensating module.
This compensating scheme is applicable to high performance multiplexed systems of any length. It is very well suited for use in intermediate length telecommunication systems that incorporate wavelength division multiplexing. An intermediate length telecommunication system is one in which regenerator spacing is in less than 100 km, for example in the range 10 km to 60 km. In such systems, the need for large negative dispersion slope is required together with an intermediate total dispersion in the range of about xe2x88x924 ps/nm-km to xe2x88x9240 ps/nm-km. Cabled compensating fiber having these properties can also be used in those systems in which complete compensation is not desirable, for example in soliton systems, or systems dominated by four wave mixing.
The present invention provides a fiber that compensates for total dispersion and total dispersion slope and furthermore is sufficiently bend resistant to allow cabling or other types of buffering. These other types of buffering are known in the art and include, for example, insertion of the fiber into a slotted member which is then jacketed.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.